1. Field of the Invention
The present invention relates to a maximum road friction force estimating device that estimates maximum road friction force which is a maximum value of road friction force caused between a road surface on which a vehicle runs and a tire of the vehicle, and to a brake torque control device utilizing this maximum road friction force estimating device.
2. Description of the Prior Arts
It has generally been well-known that road friction force (road friction coefficient) caused between a road surface on which a vehicle runs and a tire of the vehicle is changed according to a slip ratio (defined as “(vehicle body speed−wheel speed)/(vehicle body speed)” in this specification) of a tire (wheel), and that it is at a maximum when the slip ratio takes some value (for example, values within 5 to 30%) determined by the road surface or the like. Therefore, it is preferable in an ABS (anti-lock brake system) device that brake torque (accordingly, brake fluid pressure) exerted on the tire is controlled so as to keep the slip ratio of the tire to a value by which the maximum road friction force can be demonstrated, in order to achieve the shortest braking distance of the vehicle.
Therefore, an ABS device disclosed in the patent document 1 described below detects road friction force exerted on a tire and brake torque exerted on the tire respectively by a sensor and calculates a differential parameter that is a value according to a value obtained by subtracting the brake torque from the road friction force and that is a value equivalent to an angular acceleration of the tire. This device monitors the value of the differential parameter for determining that the slip ratio of the tire passes the value by which the maximum road friction force can be demonstrated, as it increases or decreases, whereby control for increasing or reducing the brake hydraulic pressure is executed by turns based upon the result of the determination. In other words, the brake torque exerted on the tire is controlled such that the slip ratio of the tire moves within the predetermined range including the value by which the maximum road friction force can be demonstrated.
[Patent Document 1]
Japanese Unexamined Patent Application No. HEI11-115722
However, in the device disclosed above, there may be a case where the slip ratio of the tire falls relatively greatly below the value by which the maximum road friction force can be demonstrated temporarily as a result of the pressure-reducing control of the brake hydraulic pressure, and hence, braking distance becomes long due to the relatively great temporal reduction of the road friction force from the maximum road friction force. Specifically, there is a problem that surplus pressure reduction of the brake hydraulic pressure may be greatly carried out.
This problem can be eliminated if the maximum road friction force that is the maximum value of the road friction force can correctly be estimated. Specifically, if the maximum road friction force can correctly be estimated, pressure-increasing and pressure-reducing controls of the brake hydraulic pressure can be more accurately executed, resulting in that the surplus pressure reduction of the brake hydraulic pressure can be prevented. Accordingly, it has been desired that the maximum road friction force that is the maximum value of the road friction force can correctly be estimated.